Your web server has been compromised. TL;DR your website will display a content blocking interstitial to visiting users, which will direct them towards a suspicious binary download. The cause of the security breach is unclear, but I would take the normal precautions: reset relevant passwords, look for suspicious code snippets on PHP/javascript files.
Google also has a nice overview for some steps you can take to clean up your site (update third party plugins, change PWs, etc.):
https://support.google.com/webmasters/answer/163634
See this security posting from WebSense for a summary of the behavior of this attack:
http://community.websense.com/blogs/securitylabs/archive/tags/Mass+Injection/default.aspx